For example: Gravemind. Was he evolving from the the first stage or did the floods merge into it?
A Proto-Gravemind (what happens to Keyes in Combat Evolved) is created by merging a bunch of intelligent Flood forms (such as humans) in order to use their collective information and intelligence (kinda like Lekgolo/Hunters in a way).
With enough of them, it will eventually become self-aware and reach critical mass, at which point it’s considered a ‘Gravemind’. So really the Gravemind encountered in the games is a collection of thousands of bodies and minds that have been leeched of information.
The Gravemind is the worst threat the modern galaxy ever faced when you consider the fact that the Flood poses a threat to all life in the known galaxy and quite possibly the universe. A graveminds lifecycle starts off as a Proto-Gravemind seen in Halo CE which was relatively young compared to the much older and larger proto-graveminds seen in Halo Wars and Halo Wars 2: Awakening the Nightmare. These ones in particular were close to becoming full fledged Graveminds judging by there sizes and as seen with the only one we’ve come across in game from Halo 2, that is considered the final stage of evolution for the Flood.
The Graveminds intelligence honestly can’t be matched when you realize that it is basically a massive super computer and everyone who falls host to the Flood results in there collective knowledge being added to the Flood collective intelligence. Then you also realize the memories of every previous Gravemind is passed on giving each one a huge amount of knowledge.The knowledge gained from the Forerunners Ecumene from the Forerunners assimilated during the Forerunner-Flood War continues to exist so long as a Gravemind does. In a wierd twisted way one can say the Floods collective intelligence is a Bio-Domain compared to the actual domain used by the Forerunners, Precursors.
Now one can assume the Flood’s evolutionary cycle ends after this right? Well unfortunately as revealed in the Forerunner Saga. The graveminds possess another stage of evolution properly named Keyminds. Now unlike the keyminds seen in Halo Wars 2, the Keyminds created during the final years of the Forerunner-Flood War were Graveminds who had consumed a planet entirely, basically imagine a planet sized gravemind and these monsters were unmatched. Ancilla created by the Forerunner Ecumene could not hope to match them as they fell to the Logic Plague and there intelligence was so vast, they were able to use the Precursor Nueral Physics aka Star Roads to a devastating degree.